This invention relates to a process for cleaning coal and similar carbonaceous solids that contain impurities in the form of pyritic sulfur and other ash-forming, inorganic constituents and is particularly concerned with upgrading raw coal by physically removing a substantial portion of these inorganic constituents.
Raw coal contains impurities in the form of inorganic, rock-like constituents which include, among other inorganic compounds, aluminosilicates, iron pyrites, other metal pyrites and small amounts of metal sulfates. Before some coals or similar carbonaceous solids containing these inorganic impurities can be used for fuel, the solids must be cleaned or upgraded to produce carbonaceous solids having a relatively high organic content and a relatively low inorganic content so that when the solids are burned or otherwise utilized they will have a relatively high Btu content, will generate relatively low amounts of sulfur containing pollutants such as sulfur dioxide and will leave relatively small amounts of unwanted ash residue, which is formed by the oxidation of the inorganic constituents during combustion. At the present time federal standards limit sulfur dioxide emissions from coal burning power plants built between 1971 and 1977 to no more than 1.2 pounds of sulfur dioxide per million Btu. A coal which meets this emission standard is commonly referred to as a compliance coal.
The conventional method for physically treating coal for the purpose of removing inorganic sulfur and other inorganic ash-forming constituents normally involves a preliminary step of classifying crushed raw coal into several size fractions: a large size fraction normally containing particles in a size range between about 2 to 6 inches and about 1 to 1/4 inch on the U.S. Sieve Series Scale, an intermediate size fraction containing particles normally ranging in size between about 1 to 1/4 inch and about 30 mesh on the U.S. Sieve Series Scale, and a small size fraction normally comprised of particles less than about 30 mesh in size. The three different size fractions are then separately treated in equipment specifically designed to handle the particular size fraction. The large and intermediate size fractions are physically cleaned by subjecting the particles to a gravimetric separation which is normally carried out at a specific gravity in the range between about 1.3 and about 1.9 in order to divide the particles into a low density, clean fraction containing a relatively small amount of inorganic constituents and a high density, dirty fraction containing a relatively large amount of inorganic constituents. The particles below about 30 mesh that comprise the small size fraction are so tiny that they take too long to separate by gravity means and therefore froth flotation is the conventional method used for separating these particles into relatively clean and dirty fractions. Gravimetric separations and froth flotation are the conventional methods of washing coal to physically clean it; i.e., to separate the low density, clean fraction from the high density, dirty fraction.
The composition of raw coal varies depending upon the part of the country in which it is mined and the particular portion of the mine from which the coal is taken. Because of the wide variance in the original composition of raw coal, the conventional method of cleaning by crushing the coal and then washing the various size fractions to separate the low density, clean particles from the higher density dirty particles will produce a clean coal of widely varying composition. Thus, in some cases the low density fraction produced from the physical washing of the coal will contain relatively large amounts of inorganic sulfur constituents and will not satisfy the federal sulfur dioxide emission standards for a compliance coal and therefore cannot be directly burned in power plants built between 1971 and 1977 that do not utilize expensive effluent scrubbing equipment. It is normally possible to remove a greater amount of the inorganic impurities and produce a cleaner product by crushing the raw coal to a finer size prior to washing. Such a procedure, however, may still not produce a clean enough low density fraction and to further liberate enough of the inorganic impurities may require grinding or crushing to a size so fine that conventional gravimetric separations can not efficiently be used to wash the resultant product. Because of the deficiencies of conventional coal cleaning techniques and the ever increasing demand for coal with a higher heating value and a lower content of pyritic sulfur and other inorganic, ash-forming constituents, the need for improved methods of physically cleaning coal is readily apparent.